The present invention relates to a process for producing a liquid nitrogen stream, a gaseous nitrogen stream, a gaseous stream which is rich in helium and a denitrided stream of hydrocarbons from a feed stream which contains hydrocarbons, helium and nitrogen.
Such a process is used particularly for processing feed streams which are constituted by liquefied natural gas (LNG) or also natural gas (NG) in gaseous form.
This process is used for new units for liquefying natural gas or new units for processing natural gas in gaseous form. The invention also applies to improving the effectiveness of existing units.
In those installations, the natural gas must be denitrided before being conveyed to the consumer, or before being stored or transported. The natural gas extracted from underground deposits often contains a significant quantity of nitrogen. It further commonly contains helium.
Known denitriding processes allow production of a denitrided hydrocarbon stream which can be conveyed towards a storage unit in liquid form in the case of LNG or towards a gas distribution unit in the case of NG.
Those denitriding processes further produce streams which are rich in nitrogen and which are used either to provide the nitrogen necessary for the operation of the installation or to provide a combustible gas which is rich in nitrogen and which serves as a fuel for the gas turbines of the compressors which are used when the process is carried out. In a variant, those streams which are rich in nitrogen are released to the atmosphere in a flare stack after the impurities, such as methane, have been burnt off.
The above-mentioned processes are not entirely satisfactory, particularly because of the new environmental constraints being applied to the production of hydrocarbons. So that the nitrogen produced by the process can be used in the production unit or released to atmosphere, it must be very pure.
The fuel streams produced by the process and intended to be used in gas turbines must, however, contain less than from 15 to 30% of nitrogen in order to be burnt in special burners which are configured to limit the production of nitrogen oxides which are discharged to atmosphere. Those discharges are produced in particular during start-up phases of the installations which are used to carry out the process, wherein the denitriding process is not yet very efficient.
For economic reasons, the energy yield of such denitriding processes must further permanently be improved. The processes of the above-mentioned type do not allow the helium contained in the natural gas extracted from underground to be exploited, though helium is a rare gas having a high economic value.
In order to at least partially overcome those problems, US2007/0245771 describes a process of the above-mentioned type which simultaneously produces a stream of liquid nitrogen, a stream which is rich in helium and a gaseous stream containing approximately 30% of nitrogen and approximately 70% of hydrocarbons. That gaseous stream which is rich in nitrogen is intended, in this installation, to form a fuel stream.
However, this process is not completely satisfactory because the quantity of pure nitrogen produced is relatively low. The fuel stream further contains a high quantity of nitrogen which is not compatible with all existing gas turbines and which is capable of generating a large number of polluting emissions.